Why Must the Flu Vaccine Be Repeated Every Year?
Unlike many other vaccines, the influenza vaccine must be given every year. The reason is that flu viruses keep changing through small mutations called antigenic drift. The WHO monitors virus circulation globally through the GISRS system and updates vaccine composition recommendations twice a year.
In four-season countries, flu cases peak in winter. But in tropical Indonesia, influenza viruses circulate year-round with significant increases during the rainy season, so vaccination can be done anytime. Most flu vaccines today are quadrivalent, meaning they protect against four virus strains at once: two influenza A and two influenza B.
Groups That Need It Most
The WHO recommends annual influenza vaccination especially for vulnerable groups: children aged 6 months to 5 years, adults over 65, pregnant women, people with chronic diseases (such as asthma, diabetes, heart disease, and kidney disease), and healthcare workers at high risk of virus exposure.
For these groups, the flu is not just a bad cold. Influenza infection can develop into serious complications such as pneumonia, middle ear infections, or worsening of existing chronic conditions — and can even be fatal. Vaccination protects yourself while suppressing virus spread in the community.
Side Effects and Who Should Be Cautious
Flu vaccines cannot cause the flu because they contain inactivated virus or only small parts of the virus. Common side effects are mild, such as pain, redness, or swelling at the injection site, and usually disappear within a day or two.
However, the vaccine is not recommended for people with severe egg allergy (since some vaccines are made using egg-based media), those currently seriously ill, those who have had an allergic reaction to a previous influenza vaccine, or those who experienced Guillain-Barre syndrome after a flu shot. Consult a doctor before vaccination if you fall into these groups.
Flu Prevention Beyond Vaccination
Vaccination is the most effective way, but other simple steps remain important: cover your mouth and nose with a tissue or your elbow when coughing and sneezing, wash hands with soap and running water for at least 20 seconds, and wear a mask when sick or in crowds.
If already infected with the flu, stay home and avoid contact with others — especially vulnerable groups — until the fever has subsided for at least 24 hours without fever-reducing medication.
